Analysis of quality characteristics of radiotherapeutic photon beams

Detta är en avhandling från Stockholm : Stockholm University

Sammanfattning: The lepton and photon contamination of high energy photon beams for radiation therapy has been investigated computationally and experimentally. The contaminating particles will effect the quality of the beam by increasing the surface dose, degrading the uniformity of the beam, broadening the penumbra, and increasing the energy imparted outside the useful beam.The surface dose due to contaminating leptons depends on the photon energy, the atomic number of the targets and filters and the geometry of the treatment head. The lepton contamination is generally high for low and high energy photon beams and has a minimum around 8 to 10 MV x-rays. The main lepton source is normally the air at low photon energies and the beam flattening filter at high photon energies.The contamination by scattered photons from the treatment head is normally below 2-3 per cent of the absorbed dose at dose maximum. The energy and angular distribution of scattered photons is rather similar to that of the primary photons so for most dosimetric purposes they may be treated as part of the primary photon beam.However, the dosimetry of photon beams will be significantly affected by contaminating leptons. In the build-up region there are large perturbation effects in parallel plate chambers that depend both on geometry and materials of the chamber and the contamination of the beam.

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